Organosilicas assemblies

Etienne, M. Walcarius, A. 2005. Evaporation induced self-assembly of templated silica and organosilica thin films on various electrode surfaces. Electrochem. Comm. 7 1449-1456. 

Ordered macroporous materials (OMMs) are a new family of porous materials that can be synthesized by using colloidal microspheies as the template. - The most unique characteristics of OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a three-dimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precursor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 30 periodicity of the materials, these structures have been extensively studied for photonic applications. In this paper, the synthesis and characterization of highly ordered macroporous materials with various compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorption/separation is analyzed and discussed. 

Many of the non-silica compositions showed problems with the stability and quality of the structure. Efforts to address these issues have been on going and quite successful in some cases such as all-alumina compositions (see below). Silica-based materials remain dominant as the most versatile and best quality molecular sieves (structure and stability) available by a facile synthesis. These attributes, especially the convenient synthesis made mesoporous silicate attractive for post-synthesis functionalization with other elements as well as organic moieties with active groups/ccnters. Recently the compositional diversity has been extended further to include both silica and organic moieties within the framework. The new class is referred to as periodic mesoporous organosilicas (PMOs). The synthesis involves surfactant-assisted assembly by hydrolysis of organo-silicon compounds. Additional discussion of the PMOs is presented below. 

The self-assembly synthesis of mesoporous organosilicas with framework and surface groups is a promising method toward obtaining nanostmctures with two opposing properties that coexist peacefully. 

Mixed micelles can be exploited as templates for preparation of nanoporous materials. Oligomeric alkyl-ethylene oxide surfactant (Polyoxyethylene (2) cetyl ether, B52), and ionic surfactant (cetyltrimethylammonium bromide, CTAB) form mixed micelles that self-assemble into well-ordered hexagonal and bimodal mesostructures. These systems can be used as templates to synthesize hydrothermally stable organized periodic mesoporous organosilicas (PMOs). The X-ray diffraction (XRD), TEM, BET, NMR and hydrothermal studies have been used to investigate the effects of B52 on the formation of various PMOs. The addition of B52 in the surfactant solution improves... 

Camarota, B. Mann, S. Onida, B. Garrone, E., Heirarchical Self-Assembly in Molecularly Ordered Phenylene-Bridged Mesoporous Organosilica Nanofilaments. ChemPhysChem 2007,8, 2363-2366. 

Regarding organosilicas, the control of the pore structure on the basis of external templating by self-assembly of surfactant molecules has been reported. The structuration of the pores was obtained only from organosilicon precursors of a small size. Provided the organic to sihcon ratio was low, self-assembled organo-sihcas with ordered pore structure can be obtained [39]. 

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